1. Field of the Invention
The present invention relates generally to an ignition timing control system for a vehicular internal combustion engine which prevents the occurrence of engine knocking.
2. Description of the Prior Art
It is desirable to prevent occurrence of engine knocking which is due mainly to accelerated violent burning of uncombusted gas since knocking brings about energy losses (reduction of engine output) and applies shocks to all parts of the engine and furthermore increases fuel consumption.
A conventional system for controlling ignition timing so as to suppress knocking is exemplified by "VG-series Engine Service Manual 1983" published by Nissan Motor Co., Ltd. on June 1983.
FIGS. 1 through 4 show the construction and action of the conventional ignition timing control system disclosed in the above-identified Japanese document.
In FIG. 1, 1 denotes an engine. Intake air is supplied to each engine cylinder via an intake air pipe 3 from an air cleaner 2 while fuel is supplied thereto via a fuel injector 4 according to an injection signal Si. Each cylinder is provided with an ignition plug 5 which receives a high-voltage pulse Pi from an ignition coil 7 via a distributor 6 at every ignition timing. The ignition coil 7, the distributor 6, and a plurality of ignition plugs 6 constitute ignition means 8 for igniting and burning the air-fuel mixture supplied to the engine. The ignition means 8 generates and discharges the high-voltage pulse Pi in accordance with an ignition signal Sp. In addition, the air-fuel mixture within each engine cylinder is ignited and exploded in response to discharge of the pulse Pi and the resulting exhaust gases are exhausted to atmosphere via an exhaust pipe 9.
In addition, the rate of flow Qa of intake air is detected by means of an air flow meter 10 and controlled by means of a throttle valve 11 installed within the intake air pipe 3. Vibrations Ve in the engine body 1 are detected by a knock sensor 12. An output signal from the knock sensor 12 is inputted to the knock vibration detector 13. The knock vibration detector 13 comprises a BPF (Band Pass Filter) which enables the passage of only a frequency range corresponding to vibrations due to knocking and an integrator which generates a voltage Vn (Vn=0 through 5 V) proportional to the intensity or amplitude of knocking vibrations generated per combustion stroke and outputs this voltage to a knock discriminator 14. The knock discriminator 14 compares the output voltage Vn of the knock vibration detector 13 to a determinating reference value Vo. If Vn&gt;Vo, the knock discriminator 14 outputs a knock determination signal Sn having a high logic level "H". If Vn.ltoreq.Vo, the knock determination signal Sn turns to the lower level "L". In addition, the engine revolutional speed N of the engine 1 is monitored by a crank angle sensor 15 built into the distributor 6. The electrical signals from the air flow meter 10, the knock discriminator 14, and the crank angle sensor 15 are inputted to a control unit 16. The control unit 16 carries out the ignition timing control on the basis of the information from the sensors described above (although the control unit 16 also controls the fuel injection amount injected by the fuel injector 4, the detailed description thereof is omitted).
FIG. 2 is a block diagram of the major functional element of the ignition timing control system. The control unit 16 comprises functionally a correction amount calculator 21, a corrector 22, an ignition signal generator 23, an ignition timing calculator 24, and a Tp calculator 25. The Tp calculator 25 receives signals from the air flow meter 10 and crank angle sensor 15 and derives the basic fuel injection amount Tp. The basic fuel injection amount Tp in units of milliseconds (since the fuel injector 4 opens to inject fuel at a fixed rate for an opening duration determined by a pulse duration) is derived from the following equation (1) and the results of this calculation are outputted to the ignition timing calculator 24: EQU Tp=KxQa/N (1)
where K is a constant
The ignition timing calculator 24 receives the signal from the crank angle sensor 15. The ignition timing calculator 24 looks up a basic advance angle value SAo from a three-dimensional table map using known table look-up techniques and outputs the basic advance angle SAo to the corrector 22. Since the basic advance angle value SAo corresponds to an optimum ignition timing according to engine operating conditions, it is represented by a crank angle value before top dead center in the compression stroke of a specific engine cylinder. The corrector 22 furthermore receives a retard angle correction value SAk from the correction amount calculator 21. The correction amount calculator 21 calculates the retard angle correction value SAk (SAk .ltoreq.0) to correct the basic advance angle value SAo toward the retardation side depending on the presence or absence of knocking. The initial value of the retard angle correction value SAk is set to 0.degree. and when the knocking discrimination signal Sn is at its high ("H") level, the value SAk is calculated from the following equation (2) for each ignition timing: EQU SAk=SAk'-.DELTA.SAr, (2)
where SAk' denotes the retard angle correction amount from the previous ignition timing, SAk a retard angle correction amount for the present ignition timing, and .DELTA.SAr a correction value toward the retardation side per ignition cycle.
On the other hand, when the signal Sn is at a low ("L") level (no knocking), the value SAk is updated in the following equation (3): EQU SAk=SAk'+.DELTA.SAa, (3)
where .DELTA.SAa denotes a correction value toward the advance side per ignition cycle. The upper limit of SAk when updating toward the advance side is zero degrees (0.degree.) and will never be a positive value exceeding zero degrees. The corrector 22 corrects the basic advance angle value SAo with the retardation angle correction value SAk and calculates a final advance angle value SA expressed in the following equation (4): EQU SA=SAo+SAk (4)
The ignition signal generator 23 outputs the ignition signal Sp to the ignition means 8 at a timing corresponding to the final advance angle SA. The high-voltage pulse Pi is generated at the same timing to ignite the air-fuel mixture.
Hence, if knocking is detected, the ignition timing is repeatedly retarded in small increments to suppress knocking and thereafter, once the knocking stops, the ignition timing is again slowly advanced to hold the optimum combustion state. In this case, the correction value .DELTA.SAr is set approximately to 1.degree. and the value of .DELTA.SAa is approximately set as follows: .DELTA.SAa=.DELTA.SAr.times.(1/10 to 1/15). The reason for this difference in value is that although knocking must be immediately suppressed, the return from the retardation angle to the normal advance angle is best carried out slowly so that the ignition timing angle does not quickly approach the knocking region again.
However, since the conventional ignition timing control system is so constructed that once knocking is actually detected, the ignition timing is retarded, it is inevitable that engine performance (e.g., of torque) is reduced at the initial stages of knocking. Furthermore, since the conventional system is so constructed that after the knocking is suppressed, the ignition timing is slowly returned to the advance side, the ignition timing may be retarded more than is necessary when knocking occurs, for example, due to a especially lean air-fuel mixture. Consequently, it is necessary to improve the conventional ignition timing control system in order to enhance engine driving performance.
In more detail, when acceleration is ordered via an accelerator pedal at a time t.sub.1 as shown in FIG. 4(a), the increase in the fuel supply lags slightly behind the change in the supply amount of intake air, e.g. the increase in the fuel supply starts at a time t.sub.2 following the time t.sub.1. This delay introduces a temporary leanness to the air-fuel mixture ratio (the air-fuel mixture becomes leaner than the stoichiometric ratio) as shown in FIG. 4(b). Consequently, this causes relatively intense knocking as shown in FIG. 4(c). This is because the combustion speed is slower for such lean air-fuel mixtures and the exhaust temperature and thus the temperature at the exhaust valve(s) are increased, so that the engine cylinder is subjected to knocking. At this time, the ignition timing starts to be corrected in a series of steps of value .DELTA.SAr toward the retardation side for each ignition timing starting at the ignition timing following time t.sub.1 as shown in FIG. 4(d). The above-described correction process continues until the knocking is suppressed at time t.sub.3 as shown in FIG. 4(c). Hence, the interval Tn between times t.sub.1 and t.sub.3 is the interval during which knocking occurs. After knocking intensity drops to an acceptable level at the time t.sub.3, the ignition timing starts to be returned toward the advance side. The interval Tn is practically limited to within several engine revolutions after the accelerator pedal is depressed. That is to say, knocking due to acceleration occurs only within the interval Tn. For convenience, Tn is referred to as a knock induction time interval. It should be noted that although during the time interval Tn, knocking actually occurs in the conventional system as shown in FIG. 4(c), the knock induction time interval also refers to an interval during which there is a possibility of inducing knocking due to a lean air-fuel mixture immediately following the onset of acceleration.
Thus, even if engine knocking is suppressed within the knock induction time interval Tn, the influence of knocking cannot be eliminated completely and engine performance is degraded at the initial stage of acceleration.
In addition, since the process of returning ignition timing toward the advance side upon completion of the knock induction time interval Tn is carried out only with small increments .DELTA.SAa, a prolonged reduction of torque injurious to the output performance of the engine 1 cannot be prevented. Therefore, the correction of the ignition advance angle needs to be carried out with an improvement in output performance in mind.